Electronic circuits are often constructed of several different functional units or modules that communicate with each other to provide some function more complex than each of the component modules could themselves perform. Simply looking at the circuit board in most electronic devices will reveal a variety of components, including multiple integrated circuits or “chips” that are the modules from which a larger circuit is constructed.
These components are typically linked by conductive copper traces on a printed circuit board, which serve to conduct signals from one portion of a circuit to another. Each chip typically performs one or more functions on received electrical signals, and outputs electrical signals to another component or to an interface such as a display. Communication of signals between components has become more complex as the speed of devices such as computer processors and memory have increased and power efficiency continues to improve. Higher signal frequencies result in a noisier electrical environment, and lower power signals are inherently more susceptible to noise or interference picked up in the conductors linking electrical devices in a circuit.
One method of reducing the influence of noise or interference in conductors, whether between circuit modules inches away or between electric devices miles apart, is to send signals as differential signals instead of as single-ended signals. In single-ended signals, a single signal line changes voltage relative to a ground or reference voltage level that remains constant. Electromagnetic interference received in the signal conductor will cause its voltage level to change, resulting in a change in the intended signal level relative to the chosen reference potential. Noise from the power source used to generate the signal is also conducted over the signal line, resulting in deviation from the intended signal voltage level.
Differential signals are sent over two signal lines, such that a positive change in voltage in one signal line is mirrored by a negative change in voltage in the other signal line. Because the difference between signal lines is detected to determine the signal state, noise picked up by both signal lines will not be detected since the interference will be essentially the same in both differential signal lines. Also, power supply fluctuations in the circuit creating the differential signal have less of an effect on the differential signal due to the construction of typical differential driver circuits. The advantages of differential signal drivers are therefore substantial in a noisy or high-speed signal environment, but are offset somewhat by the added complexity of the differential driver circuit and the need for two signal lines to carry the differential signal.
It is therefore desired to retain advantages of differential signaling while using a single signal conductor.